The applicability of cyclodextrins in chromatographic separation and purification processes were previously described at length in reviews by W. L. Hinze, Separation and Purification Methods, 1981, 10(2), 159-237; Y. Kawaguchi, et al., Anal. Chem., 1983, 55, 1852; D. M. Armstrong, et al., Anal. Chem., 1985, 57, 234 and S. Li, et al., Chem. Rev., 1992, 92, 1457. Chromatographic separation on chiral stationary phases (CSP) is also the most convenient analytical method for the determination of enantiomeric purity (see for example S. G. Allenmark, Chromatographic Enantioseparations: Methods and Applications, 2nd ed., Prentice Hall, N.J., 1991). In recent years, tremendous research efforts were made in bonding cyclodextrins to solid matrices, such as silica gel, via amino or amido linkages. However, these bonds were inherently unstable to hydrolysis, thus placing severe limitations on their use in aqueous media. Alternative approaches for immobilizing CD using hydrolytically more stable ether linkages (U.S. Pat. No. 4,539,399) or carbamic acid moieties (U.S. Pat. No. 503,898) were also investigated. However, in all these approaches, arising from the presence of multiple hydroxy moieties in the CD starting materials, regioselective derivatisation of cyclodextrin cannot be readily effected. Thus, reaction may take place on the 2, 3 or 6- position of cyclodextrin and may result in mixtures of multi-functionalised CDs instead of the desired regiodefined compound.
It was often reported that derivatized CD stationary phases show definite enantioselectivity for a variety of compounds while pristine cyclodextrin bonded LC stationary phases depict low enantioselectivity. Thus, as an example, enantioselectivity of the materials were generally increased with increasing degree of derivatisation of the --OH groups on CD with carbamate groups on cyclodextrin as of an increasing surface concentration of the functionalised cyclodextrin immobilized on the support materials (D. W. Armstrong et al., Anal Chem., 1990, 62, 1610; T. Hargitai et al., J. Chromatogr., 1993, 628, 11; T. Hargitai, et al., J. Liq. Chromatogr., 1993, 16(4), 843). In order to maximise the extent of cyclodextrin derivatisation, large molar excesses of derivatizing reagents under vigorous conditions were often used. However, as the derivatisation processes invariably involved the prior immobilization of underivatised cyclodextrin on the support material followed by functionalisation procedures involving solid-liquid phases, partial derivatisation of the hydroxyl groups of the cyclodextrin usually resulted with large, sterically encumbered substituents consistently having a lower extent of derivitisation. In addition, these methods did not give good reproducibility or uniformity of product with the consequence that separation of enantiomers may vary from batch to batch of the obtained CD-based CSP.